The invention relates to electronic circuitry for use in telephone subscribers' instruments.
In our patent application P. F. Blomley et al., Ser. No. 662,487, now U.S. Pat. No. 4,608,462 (P. F. Blomley et al., Ser. No. 626,004, now abandoned) we have described a set of semiconductor chips for use in a telephone subscriber's instrument, and one of the chips described therein is usable on its own when the telephone only has to provide plain ordinary telephone (POT) service, and is usable with one or more other chips when other facilities (e.g. loudspeaking telephone) are needed. Even when used on its own the chip has a number of circuit units not all of which are in use at any one time. Hence consumption of electrical power may be rather greater than is desired. It is an object of the invention to overcome this disadvantage.